What is colour temperature – and why does it matter when buying lightbulbs?
A bulb is a bulb, right? Wrong. Where once you only needed to choose between bayonet and screw, you must now consider how bright it is and the tone of light a bulb puts out – alongside the fit. Most important among these is its colour ‘temperature’, which can make an enormous difference to the feel of a room, and even how well you work, rest and play.
Colour temperature, expressed in degrees Kelvin and shortened to K on the neck of a bulb, measures a light’s hue, on a scale from orange to blue. Candlelight, which is fairly orange, is between 1000K and 1500K, and a bright blue sky sits at about 15,000K. Floodlights, nightlights and the screen of your Kindle sit somewhere in between.
Household bulbs generally cover 2000K to 4000K. So, a ‘soft’ or ‘warm’ white bulb will be between 2500K and 2700K; a ‘cool’ white bulb between 3000K and 4500K, and a ‘daylight’ bulb around 6500K. The terms ‘daylight’ and ‘full spectrum’ are interchangeable here, so whichever you buy will produce the same kind of light.
Does it really matter?
Choosing the right colour matters. Evolution has made us reactive to light. During our cave-dwelling period, we would have woken with the sun when the sky was cooler – and bluer – and gone to bed after hours of staring at amber embers in a fire. So, our brains grew accustomed to being alert, ready to hunt and defend ourselves in the presence of cool light, and to slowly shut down when bathed in warmer tones.
Bearing this in mind when buying lights for your home can enhance your wellbeing. Warmer tones in the bedroom will help promote sleep, so choose a bulb rated at 2700K or less. In the kitchen or office, opt for something cooler, to keep you alert and help you see what you are doing; 3500K to 4000K should be ideal. If you are a crafter, knitter, artist or photographer, opt for a daylight bulb, which will help you see the most accurate tones in your work.
Colour temperature wasn’t such a consideration with incandescent bulbs, which generally glow between 2700K and 3000K. It is little surprise, then, that the first low-energy bulbs, with their comparatively cooler light, were considered ‘harsh’ and somewhat unappealing. Now that the technology had bedded in, low-energy bulbs cover the full spectrum and have usurped traditional filaments in most homes.
Temperature versus brightness
Don’t assume that a daylight bulb (6500K) will necessarily be any brighter than a ‘warm’ one (2700K). Colour temperature only determines the character of the light, and although cold light can feel superficially brighter, that is just another quirk of evolution. Brightness isn’t measured in Kelvin, but lumens.
Incandescent bulbs were rated according to Wattage – or how much energy they used. Watts are still quoted on CFL and LED bulb packaging and you can, to a degree, convert them to their incandescent equivalences (indeed, some packaging will also state a ‘Wattage equivalent’ figure to save you the job). If you are replacing a 100W bulb, buy an LED or CFL element of around 1600 lumens. For a 60W bulb replacement, opt for 800 lumens and, for 40W, 450 lumens. You can buy any of these at a range of colour temperatures, so you could have a bright, warm light in your bedside lamps just as easily as a dim, cool light hanging from a ceiling rose.
Putting it all together
When fitting lightbulbs first consider the type of bulb you are replacing, then the quality of light you want.
If switching from incandescent to CFL or LED, convert the old bulb’s wattage to lumens. If you are already using a low-energy bulb, check its current rating and look for the same in its replacement. Then consider the kind of light you want: warm in the bedroom and lounge, cooler in rooms where you are active. To recreate the kind of light produced by an incandescent globe, look for something between 2700K and 3000K.
If all of this has set your head spinning, investigate a ‘tuneable’ light, like the Wi-Fi bulbs from Philips’ Hue and Ikea’s Trädfri ranges, or the inexpensive remote-controlled bulbs you can pick up online. These offer the best of both worlds, allowing you to both trim the intensity (luminosity) and adjust the temperature of your light on the fly. They simulate an infinite range of bulbs in a single unit and save you the task of choosing between them.